Catalysts are used in a variety of industries and processes to control the rates and/or pathways of chemical reactions. In some instances, catalysts are used to control electrochemical reactions, and such catalysts are referred to as electrocatalysts. The catalyst materials of the present invention may be used in electrochemical and nonelectrochemical processes; however, their high electrical conductivity, stability and resistance to poisoning makes them very useful in electrochemical processes, as for example in fuel cells.
Catalysts frequently are used in harsh environments, and should be chemically stable so as to maintain their reactivity. In those instances where catalysts are used in electrochemical processes, they should also have good electrical conductivity. Noble metals are frequently employed as catalysts, and as generally understood, such metals include platinum, palladium, osmium, iridium, gold, ruthenium and rhodium. In general, the noble metals have good electrical conductivity and are relatively inert; however, such materials are very expensive; consequently, they are often disposed on a support member. The support member should be chemically stable in harsh environments, and it should have good electrical conductivity in those instances where the catalyst is being used electrochemically.
In many instances, noble metal catalysts are supported on carbon. Carbon is fairly inert and low in cost; but, the electrical conductivity of carbon is not sufficiently high for many purposes. In addition, carbon does not have good mechanical integrity, and is reactive under certain chemical conditions, particularly highly oxidizing conditions. Carbon supported catalysts are known in the prior art, for example as shown in U.S. Pat. Nos. 5,183,713; 5,024,905 and 4,677,092, and as disclosed therein, such catalysts have been used as electrodes in fuel cells.
In some instances, catalysts are supported on ceramic materials; however, ceramics are generally of very low electrical conductivity, which limits the use of the catalysts. PCT publication WO 92/16027 shows noble metal catalysts supported on tungsten oxide. U.S. Pat. No. 4,868,841 shows a catalyst body used in carbon dioxide lasers, and comprised of a noble metal catalyst which is supported on an electrically conductive material such as silicon carbon or tin oxide. U.S. Pat. No. 5,705,265 shows catalysts comprised of a coating of tin oxide supported on a non-conductive substrate, and further including a noble metal in the coating. These supports have fairly high electrical resistivities, and are used as resistive heating elements for raising the catalyst to a desired working temperature. The disclosed materials are not electrocatalysts, and in general, the electrical conductivity of the disclosed supports is too low to allow these materials to be used as catalysts in electrochemical devices such as fuel cells.
In some instances, fuel cells are operated on hydrogen which is produced by the reformation of hydrocarbon on alcohol fuels; and such hydrogen is often contaminated with CO, which has been found to be a poison for many prior art catalysts. In other instances, fuel cells are operated on methanol, and CO poisoning is a problem in methanol cells also. Reformed fuels and methanol are good sources of energy for fuels cells, and there is thus a significance interest in fuel cell catalysts which are CO tolerant.
There is thus a need for a catalytic material which is stable under a wide range of operating conditions, resistant to CO poisoning, and which has good electrical conductivity and sufficient mechanical integrity to allow it to be used in applications such as fuel cells. In addition, the material should be relatively easy to fabricate and low in cost. As will be explained in further detail hereinbelow, the present invention provides a catalyst which is low in cost, stable and has good electrical conductivity.